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10-24-2018, 07:23 PM | #121 | |
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Re: Trigger Mechanism Design Rules
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Last edited by popgun pete; 10-24-2018 at 09:08 PM. |
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10-25-2018, 09:48 PM | #122 |
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Re: Trigger Mechanism Design Rules
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10-26-2018, 03:32 AM | #123 | |
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Re: Trigger Mechanism Design Rules
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That is interesting … I would have thought that a C shape would be stronger than a square cut because there are no sharp corners that can propagate a weakness. I did some stress analysis testing on a computer and I always found that the weakest point in the sear shaft is the area between the shaft sear pin and the shaft sear and not the notch itself. Obviously it would be different from mech to mech as they have different measurements and different angles of attack with regards to how the spear shaft sits on the notch. In the test study, I was able to dramatically improve the stress analysis data by simply adding a bigger fillet on the right angle between the shaft sear back stop and the shaft sear. You can get away with a 2mm fillet and maybe even a 2.5mm fillet before touching the spearshaft back end. That little tiny change dramatically increased the strength of the shaft sear. |
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10-26-2018, 03:49 AM | #124 |
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Re: Trigger Mechanism Design Rules
Hi Spearq8,
When overpowered to destruction, the part that destructs is not the shaft. I agree, a square will have sharp edges and be weaker than a C shaped, but the fail is not the shaft. The sear in the mechanism itself distorts and or, bends the sear pivot pin. Our "square" sear notch in the spear does have rounded corners. The degree of glass content in the glass filled nylon cassette also plays a huge part in the mechanisms "hold-ability" as does the corners in the roof.
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10-26-2018, 08:17 PM | #125 |
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Re: Trigger Mechanism Design Rules
The squarer shaft tail notch probably engages lower on the tooth. If you looked at a fully loaded up spear tail and sear tooth of the square cut type you would see that they would connect low on the tooth as the sear lever under extreme pressure tilts forwards slightly (1/4” sear lever pivot pin). Bill Kitto found that when he applied thousands of pounds of spear pull the slightly tilted tooth was torn off the sear lever by the spear tail which cut through the tooth, however these are not spearfishing loads and are for test purposes only. One way to lift the point of destruction higher was to square the flat faces up to each other so that the force is evenly spread which in turn meant that an over pushed sear lever for mechanism relatching could not be tolerated, over pushing being used in the heavy duty M3 Mechanism. Kitto developed the M5 two-stage locking derivative of the M3 for this very purpose, however anyone wishing to push these “MBT” trigger mechanisms to their limits will be handling a gun with scary band loads. One spearfishing identity, whom I will not name, put aside his vastly overpowered M Mechanism cannon when the creaking and groaning emanating from the laminated timber weapon had him in fear of his life and he quickly abandoned any plans to shoot one monster with another.
Note that heavy side plate trigger mechanisms are for ballasting purposes in extra heavy duty or "Main Battle Tank" underwater weapons. Last edited by popgun pete; 10-28-2018 at 06:19 PM. Reason: added photos of M5 "MBT" two-stage mech (Kitto's images) |
10-28-2018, 06:02 PM | #126 | |
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Re: Trigger Mechanism Design Rules
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Last edited by popgun pete; 10-28-2018 at 06:21 PM. |
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11-25-2018, 07:17 PM | #127 |
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Re: Trigger Mechanism Design Rules
An old email which may be of interest here.
I spoke with Wally Gibbins many years ago and asked him about his trigger mechanism design and the mechanisms that were contemporaneous with it, both at the time and before. Wal actually documented it all later in a personal letter to me which I later had published posthumously by Wal thanks to Barry Andrewartha of IF&SN so that Wal could speak directly to the “brotherhood” and in a sense, in his farewell salutation, “sign off” for a last and final time. In the old days they would have regarded an Omer “Cayman” as a childlike weapon no better than a toy, being of a flimsy construction that would not last a week, much less a season. Of course that would be judging it by the plastic guns of the time, the feeble and simplistic holidaymaker's guns from the Europeans that shot fork head tridents on top of skinny shafts and were a low cost option to their more serious weapons such as the legendary “Champion Arbalete” with its socket head muzzle, preferably the version with four sockets. A gun was expected to use either 5/8” or 3/4” solid bands of gum rubber and either a 5/16” or 3/8” diameter shaft just in case any sharks had to be seen off with a shot driven through them. The square notch spear tail is a direct carry lover from the single-piece trigger, as any significant angle on the tooth and the mechanism shoots without you pulling the trigger. The angled sear tooth used by the Europeans is a direct carry over from the pull down sear, spring gun mechanism, as with a square cut tooth pulling the trigger would otherwise drive the shaft back against the spring loading/band pull. Once cam lock mechanisms were created they were two state, i.e. either cocked or discharged, and why divers like them is that they can be “dry fired” to check the latch and release action on the spear without any band load. As timber guns of any reasonable length (in excess of a metre) are going to have some depth to the stock, the “real estate”already being there it makes sense to fill it up with the mechanism. A sear box is not only held in with the transverse pins, its forward edge presses hard on the stock, hence a deep mechanism case transfers its load into the stock by butting squarely into the pocket cut for it in the timber. You can see this very clearly in the Riffe Metaltech as the sear case in its plastic isolation cradle butts directly to the stock. The reason why Wally Gibbins made his reverse mechanism, seen in the “Sea Hornet” and “Biller” in somewhat economy versions, is that he wanted that two state mechanism with a distinct “cock” and “release”, in fact the spear could cock the mechanism with a “slight push on it with one finger” was a familiar phrase used by Wal. I have used that trigger mechanism, in its all metal case configuration, ever since I started diving nearly fifty years ago, with the Undersee mechanism being used in my long range cannons until I replaced the sear pin “rivet” with something more substantial, just as Jay Riffe did with his own version of the Undersee which is fully made from stainless steel. |
12-19-2018, 08:47 PM | #128 |
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Re: Trigger Mechanism Design Rules
Some history which I recovered from an old computer hard disk. Here is the model in clear plastic which Mori created that eventually became the Kitto M6 through to the M8 multi-lever trigger mechanism, now used for some years in the latest JBL timber (woody) spearguns. At the time Mori was working on a mechanism cassette/sear box housing and figuring out the auto-line release action in terms of which lever to drive it off.
When I first saw Mori's model I was concerned that it was a hooking mechanism and not a cam lock as the locking lever does not roll into the trigger, instead they are one and the same thing. However time has proven that it was not going to be a problem provided all the levers are fully engaged when locked. At the time I was working on a four pivot pin, two stage mech, but thought the close set and "lean on each other" levers could be used in a four lever mech using the general shapes of Mori's levers. The parts ended up being too small and the demands on precision were great, so this "fourpin" mech never got beyond some plastic models to check it out, aided by the addition of “timing bumps” on the levers to “short circuit” the cocking or relatch action. |
02-23-2019, 05:49 PM | #129 | |
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Re: Trigger Mechanism Design Rules
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KABOOM spearfishing at maximum power! |
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03-26-2019, 11:15 PM | #130 |
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Re: Trigger Mechanism Design Rules
The double roller trigger mechanism is designed for a smooth release by eliminating “stump jumping” off the sear tooth as spear tails pop free of the reverse trigger mechanisms, however there is a simpler fix by extending forwards the sear box roof on these now rather open mouth reverse trigger mechanisms. To accommodate an increased wishbone draw the forward sear box roof extension can be slotted to allow spear tails to be driven from further rearwards by accepting the insertion of the shark fin tab. To illustrate this point I show a mechanism modified accordingly. I originally designed this top slotted mechanism for Doug Battersby, but he never got around to using it. The top slot also functions as an anti-shaft rotation or rolling device. (Doug Battersby was a member of the Rollergun Group that included myself, John Warren, Nikko Brummer (founder), Memo and Franceso, but my memory fails me on the others, although the group was always small in number, but big in ability, knowledge and spearfishing experience).
The concept of a slotted sear box case belongs to Australian spearfishing great Wally Gibbins, only in his case the shaft tail tab was on one side (the right) and so was the slot. This system was used on his very early Carbine style shoulder gun and a similar slot was in the muzzle hoop to allow the side lug to escape which was also the shooting line attachment to the shaft. Australians in the early days needed powerful guns to take care of sharks, of which there were plenty, but "thanks" to the actions of the green movement those numbers, which had been whittled down ruthlessly in the past, have been allowed to grow alarmingly again. European guns in the early days, and that included spring guns of 2 meters, lacked the sheer grunt to transfix a shark that needed to be seen off quickly, and hence the desirability of 3/8" diameters shafts in Australian waters and the guns with the necessary pep to propel them. Wally's shoulder gun used racecourse starting gate rubber for twin 3/4" bands, so that delivered the necessary horsepower. Last edited by popgun pete; 03-27-2019 at 06:29 AM. |
03-27-2019, 06:17 AM | #131 |
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Re: Trigger Mechanism Design Rules
As a matter of interest here is a sketch from Wally showing the side-slotted sear box and rear tabbed shaft. Note that this is prior to the use of line slides and is an advancement on tying shooting lines to just behind the spear tip as was done previously. In the lower sketch you can see the two band muzzle and the side slotted muzzle hoop that let the shaft tail tab through which carried the shooting line. The spear was fabricated from stainless steel with the shark fin-like tail lug welded on and is a slice of spearfishing history that would otherwise have been forgotten.
The historic photo shows Wal after having bagged a big one with his trusty shoulder gun which was constructed with all home-made parts in that nothing used in it came from mass production speargun parts, the gun in its earlier versions predating all modern band spearguns. Last edited by popgun pete; 04-03-2019 at 02:13 PM. Reason: Added a photo of a couple of Wal's guns on display some years ago |
04-02-2019, 06:59 PM | #132 |
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Re: Trigger Mechanism Design Rules
The Seatec eurogun has been around for some years now and has a very unusual trigger mechanism in that it has three elements; a sear lever, an idler wheel and a trigger each with their own pivot pin. At first glance this type of gun would be expected to use a cam lock trigger mechanism, but here Seatec have turned the arrangement into a hooking mechanism. That could have been a disaster as hooking mechanisms don't actually cam lock being more like a geared single piece trigger. In this mechanism the idler wheel reverses the push direction on the trigger, instead of compressing the trigger it tries to stretch it which is not a great idea. The idler wheel has no reduction gearing as the force in and force out radial lengths are much the same. Despite being a long mechanism the locking circle is rather small as the circle is set by the idler wheel to trigger pivot pin spacing. What saves this mechanism is the hooking contact and trigger pivot force action line run slam bang right on the periphery of the intermediate circle (shown in green) and the long trigger provides a lot of leverage. It could be argued that the layout gives a very long trigger and there is the usual gearing in the sear lever and the resultant humpy back grip housing is reasonably attractive.
Hooking trigger mechanisms need a good safety and in this trigger mechanism a sliding block moves in from the rear and catches on the back of the trigger. For the trigger to override the safety it would have to break the plastic block that is moved forwards by an external slide button. On this type of trigger mechanism you need to make sure that you use it as it also pushes the trigger closed and makes sure that it is hooking the idler wheel as otherwise the safety seems to jam and not slide. Remove this safety at your peril! Last edited by popgun pete; 04-02-2019 at 07:08 PM. |
04-02-2019, 11:01 PM | #133 |
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Re: Trigger Mechanism Design Rules
Where an idler wheel or reversing lever would be useful is for the lower trigger mechanism in a twin spear "over and under" speargun. I have just hacked around the “Seatec” mechanism to give a rough idea of how such an “upside down” mechanism would look. There is a problem with the biasing springs if only one is to be used in addition to the torsion spring already on the idler wheel, however a compression spring behind the trigger would be one solution as it could be installed in the handgrip body similar to the trigger spring used in a JBL "Magnum" trigger mechanism. The advantage of using the center lever is the resultant trigger mechanism is of the cam lock type whereas most such guns use a hooking mechanism.
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04-03-2019, 02:10 AM | #134 | |
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Re: Trigger Mechanism Design Rules
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Great to see you discussing this trigger here. I was planning on doing that myself but you did a better job than I would have done. It took me some time of head scratching to understand why exactly it feels so smooth. Eventually I realized that the length and lever of the trigger caused the smooth release. It was designed by Valerio Grassi who worked as a gun and rifle designer for Baretta. My Seatec Snake speargun that’s using this trigger is only a 65. I wonder if the larger guns where more force is applied on the trigger shoot just as well? In CMAS target shooting competitions it’s mandatory to use commercially available spearguns. Custom guns are not allowed I believe. The Seatec Snake is often seen in competition because of its great accuracy. I have the same experience and the range and accuracy of my little 65 is incredible. I believe the trigger contributes to that. |
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04-03-2019, 02:42 AM | #135 |
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Re: Trigger Mechanism Design Rules
The trigger arm leverage can be assessed by looking at the length of the red line from the trigger pivot pin to the release notch on the idler wheel and the length of the orange line from the trigger pivot pin to the place where your finger pulls-on the trigger. In the attached diagram I have added the circles at these two radii in red and orange respectively. The trigger does not have far to swing at the retention notch, but the trigger tip swings circumferentially further being at a greater radius from the pivot pin.
There are two versions of this trigger, the gold handle as shown with all metal parts and a silver version with plastic parts for the idler wheel and trigger. Last edited by popgun pete; 04-03-2019 at 03:00 AM. Reason: Added a photo |
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